1. Field of the Invention
This invention relates to electrophotographic sensitive mediums and, more particularly, to an electrophotographic sensitive medium having an improved intermediate layer provided between an electroconductive support and a sensitive layer and to various kinds of apparatus for use with this medium.
2. Description of the Prior Art
Ordinarily, to maintain a certain image density and to prevent occurrence of fogging when an image is formed by using a Carlson type electrophotographic sensitive medium after repeatedly electrically charging and exposing the medium, it is important to improve the stability of a dark portion potential and a light portion potential.
Some techniques have, therefore, been proposed which relate to improvements in the performance of injection of charge from the support into the sensitive layer, in adhesion between the support and the sensitive layer, in application of the material for forming the sensitive layer, and in providing an intermediate layer between the support and the sensitive layer to cover defects on the support.
A type of electrophotographic sensitive medium has also been proposed which has a laminated structure in which the functions of the sensitive layer are distributed to a charge generation layer and a charge transport layer. Ordinarily, the charge generation layer has a very small thickness of, for example, about 0.5 .mu.m. There is, therefore, a possibility of a considerable reduction in the uniformity of the thickness of the charge generation layer due to defects, contaminations, scratches or the like on the support surface, or foreign materials attached to the surface.
Non-uniformity of the thickness of the charge generation layer makes the sensitivity of the sensitive medium uneven. It is therefore desirable to maximize the degree of uniformity of the charge generation layer.
For these reasons, the provision of an intermediate layer has been proposed. The intermediate layer is to be formed between the charge generation layer and the support and is to have the functions of a barrier layer and a bonding layer while covering defects on the support.
The following materials are known as a material for forming the layer formed between the sensitive layer and the support: polyamide (Japanese Patent Laid-Open Nos. 48-47344 and 52-25638), polyester (Japanese Patent Laid-Open Nos. 52-20836 and 54-26738), polyurethane (Japanese Patent Laid-Open Nos. 49-10044 and 53-89435), casein (Japanese Patent Laid-Open No. 55-103556), polypeptide (Japanese Patent Laid-Open No. 53-48523), polyvinyl alcohol (Japanese Patent Laid-Open No. 52-100240), polyvinyl pyrrolidone (Japanese Patent Laid-Open No. 48-30936), vinyl acetate-ethylene copolymer (Japanese Patent Laid-Open No. 48-26141), maleic anhydride ester polymer (Japanese Patent Laid-Open No. 52-10138), polyvinylbutyral (Japanese Patent Laid-Open Nos. 57-90639 and 58-106549), tetra ammonium salt containing polymer (Japanese Patent Laid-Open Nos. 51-126149 and 56-60448), ethyl cellulose (Japanese Patent Laid-Open No. 55-143564), and the like.
However, for electrophotographic sensitive mediums using the above-mentioned materials as the intermediate layer, it is difficult to obtain potential characteristics and image qualities stable over wide environmental conditions from a low-temperature/low-humidity condition to a high-temperature/high-humidity condition, because the resistance of the intermediate layer varies with the changes in temperature/humidity.
For example, if the sensitive medium is repeatedly used under a low-temperature/low-humidity condition which increases the resistance of the intermediate layer, a substantial amount of charge remains in the intermediate layer to increase the light portion potential and the residual potential, resulting in a copied image that is fogged. If this sensitive medium is used under the same condition with an electrophotographic printer which effects reversal development, the image density is reduced or the copies obtained are not uniform in image qualities.
In addition, in a high-temperature/high-humidity condition, the barrier function deteriorates due to a reduction in the resistance of the intermediate layer, and the rate of carrier injection from the support is thereby increased, resulting in a reduction in the dark portion potential.
In a high-temperature/high-humidity condition, therefore, the density of the copied image is reduced and, if the sensitive medium is used in an electrophotographic printer which effects reversal development, the copied image tends to be damaged by black-spot defects and fogging.